The "damage" tag used for tridents does nothing. Ideally it should dictate the exact amount of damage the trident deals.

Test Data:

In survival or adventure mode, run the following commands and see how much damage each trident does:

/summon trident ~ ~5 ~ {pickup:1b,damage:2d}

/summon trident ~ ~5 ~ {pickup:1b,damage:5d}

/summon trident ~ ~5 ~ {pickup:1b,damage:10d}

Expected Results:

The tridents deal 2, 5 and 10 damage (1, 2.5 and 5 hearts) respectively.

Actual Results:

Each trident deals 8 damage (4 hearts)

Additional Information:

Naturally spawned tridents always have a damage value of 2. This is the same as the default for an arrow. If the bug is fixed, the default damage value would need to be changed to 8 to keep the trident at its usual strength.

The trident also has the crit tag from arrows, but I haven't yet tested whether this is functional.

Although I only posted it during 1.13.1, this issue has likely been in the game since tridents were first added.